NMR experiments that measure multidimensional correlations of relaxation and diffusion properties like T1-T2, D-T2 have been established as valuable methodologies for the identification of molecular species and understanding their dynamics. These experiments are similar to multidimensional NMR spectroscopic methods but involve the inverse Laplace transformation (or other inversion techniques) to study relaxation and diffusion properties which are associated with exponential decays. The advantages of such a multidimensional approach over the use of one dimensional T1 and T2 relaxation times or diffusion properties has been clearly shown.
The complete frequency dependence of spin-lattice relaxation time T1 can be measured using the field cycling relaxation technique. This is generally done in an electromagnet whose field is controlled by the current passed through the coil. These experiments are technology and hardware intensive. While field cycling relaxometry is a powerful method to measure relaxation dispersion, some of its disadvantages include the complexity of the technology and associated hardware, the field inhomogeneity of the electromagnets and the impracticality of measuring systems in-situ, especially in a space-constrained downhole environment typical of well logging applications.
Recently, the T1-T1 correlation between T1 distributions at two different Larmor frequencies was studied in a novel way using fast-field cycling relaxometry. This technique enables the study of correlations between different components of the relaxation distributions at each frequency, which is subsequently plotted and well highlighted in a two-dimensional plot. The pulse sequence for measuring the T1-T1 correlation is shown in FIG. 1A and the corresponding results on a heavy crude oil sample is shown in FIG. 1B. The disadvantage is that such experiments can be carried out only in field cycling relaxometry instruments which provide for variation of the magnetic field B0 to obtain the frequency (or magnetic field) dependence of the T1 relaxation time. Downhole NMR tools and core analysis magnets generally employ a magnetic assembly that provides a static magnetic field B0 that operates at a single frequency, making it impossible to carry out these experiments without major instrumental changes. In downhole NMR tools, due to the magnetic field gradient away from the tool, different slices resonate at different frequencies making possible multi-frequency experiments. The disadvantage of performing these experiments in a downhole NMR tool is that the different frequency measurements are made at different positions in space and thus would be difficult to interpret when formation heterogeneities exist. Also the experiments can be measured only at frequencies to which the antennas are tuned.
T1-T2 correlation experiments have been regularly carried out in the oil and gas industry as they give information about correlations between Larmor frequency and low frequency dynamics. Thus fluids which are motionally narrowed have T1=T2 and would appear on the diagonal line. But fluids which exhibit motions at or below the Larmor frequency like heavy oils or oils with asphaltene in them would show dispersion at these frequencies. The fluids that exist in small pores would also be slowed down by the interaction with the surfaces of the confining pores and exhibit slow motions. The presence of these slow motions results in these fluids exhibiting signals that are off the diagonal (as T1>T2). For example the bitumen found in organic shale has a T1-T2 ratio of 6 to 10 while heavy oils with asphaltene have T1-T2 ratios from 1.5 to 3.5, while bulk water and light oils have T1-T2 ratios that range from 1 to 1.5.
The spin lattice relaxation time in the rotating frame T1ρ is an alternative method to study the relaxation behavior as a function of frequency. The pulse sequence for measuring T1ρ at one particular frequency is given in FIG. 2. It involves a 90 degree pulse in the x direction (labeled 90x) followed by a spin lock pulse in the y direction (labeled SLy). The magnitude of the spin lock pulse dictates the frequency ω1 of the spin lock field. The magnitude of the spin lock pulse (and thus the frequency ω1 of the spin lock field) is fixed over the suite of NMR measurements. The duration (labeled τR) of the spin lock pulse can be varied over the suite of NMR measurements. The NMR signal (labeled AQy) that follows the spin lock pulse for each pulse sequence of the suite is processed to obtain a T1ρ value.